# Earliest evidence of hominin bipedalism in Sahelanthropus tchadensis

## TL;DR

This study provides early evidence that Sahelanthropus tchadensis, an ancient hominin, walked upright, based on limb bone proportions and hip/knee adaptations.

## Contribution

The study identifies bipedal adaptations in Sahelanthropus tchadensis through limb proportions and a femoral tubercle unique to bipedal hominins.

## Key findings

- Sahelanthropus limb bones show chimpanzee-like shape but hominin-like proportions.
- Hip and knee features suggest hominin-like function.
- A femoral tubercle, unique to bipedal hominins, was identified.

## Abstract

Bipedalism is a key adaptation that differentiates hominins (humans and our extinct relatives) from living and fossil apes. The earliest putative hominin, Sahelanthropus tchadensis (~7 million years old), was originally represented by a cranium, the reconstruction of which suggested to its discoverers that Sahelanthropus carried its head in a manner similar to known bipedal hominins. Recently, two partial ulnae and a femur shaft were announced as evidence in support of the contention that Sahelanthropus was an early biped, but those interpretations have been challenged. Here, while we find that both limb bones are most similar in size and geometric morphometric shape to chimpanzees (genus Pan), we demonstrate that their relative proportion is more hominin-like. Furthermore, we confirm two features linked to hominin-like hip and knee function and identify a femoral tubercle, a feature only found in bipedal hominins. Our results suggest that Sahelanthropus was an early biped that evolved from a Pan-like Miocene ape ancestor.

Limb bones of the earliest known hominin, Sahelanthropus, are chimpanzee-like in shape but demonstrate adaptations for bipedalism.